[gcc r15-1297] c++/modules: export using across namespace [PR114683]

[Jason Merrill]

https://gcc.gnu.org/g:609764a42f0cd3f6358562cab98fc220d3d2d9fd

commit r15-1297-g609764a42f0cd3f6358562cab98fc220d3d2d9fd
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jun 12 18:24:35 2024 -0400

    c++/modules: export using across namespace [PR114683]
    
    Currently we represent a non-function using-declaration by inserting the
    named declaration into the target scope.  In general this works fine, but in
    the case of an exported using-declaration we have nowhere to mark the
    using-declaration as exported, so we mark the original declaration as
    exported instead, and then treat all using-declarations that name it as
    exported as well.  We were doing this only if there was also a previous
    non-exported using, so for this testcase the export got lost; this patch
    broadens the workaround to also apply to the using that first brings the
    declaration into the current scope.
    
    This does not fully resolve 114683, but replaces a missing exports bug with
    an extra exports bug, which should be a significant usability improvement.
    The testcase has xfails for extra exports.
    
    I imagine a complete fix should involve inserting a USING_DECL.
    
            PR c++/114683
    
    gcc/cp/ChangeLog:
    
            * name-lookup.cc (do_nonmember_using_decl): Allow exporting
            a newly inserted decl.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/using-22_a.C: New test.
            * g++.dg/modules/using-22_b.C: New test.

Diff:
---
 gcc/cp/name-lookup.cc                     |  5 ++---
 gcc/testsuite/g++.dg/modules/using-22_a.C | 24 ++++++++++++++++++++++++
 gcc/testsuite/g++.dg/modules/using-22_b.C | 13 +++++++++++++
 3 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index 71482db7b766..b57893116ebc 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -5316,14 +5316,13 @@ do_nonmember_using_decl (name_lookup &lookup, bool fn_scope_p,
       /* FIXME: Handle exporting declarations from a different scope
 	 without also marking those declarations as exported.
 	 This will require not just binding directly to the underlying
-	 value; see c++/114863 and c++/114865.  We allow this for purview
-	 declarations for now as this doesn't (currently) cause ICEs
+	 value; see c++/114683 and c++/114685.  We allow the extra exports
+	 for now as this doesn't (currently) cause ICEs
 	 later down the line, but this should be revisited.  */
       if (revealing_p)
 	{
 	  if (module_exporting_p ()
 	      && check_can_export_using_decl (lookup.value)
-	      && lookup.value == value
 	      && !DECL_MODULE_EXPORT_P (lookup.value))
 	    {
 	      /* We're redeclaring the same value, but this time as
diff --git a/gcc/testsuite/g++.dg/modules/using-22_a.C b/gcc/testsuite/g++.dg/modules/using-22_a.C
new file mode 100644
index 000000000000..9eca9dacb46c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-22_a.C
@@ -0,0 +1,24 @@
+// PR c++/114683
+// { dg-additional-options "-fmodules-ts -Wno-global-module" }
+
+module;
+
+namespace std
+{
+  inline namespace __cxx11
+  {
+    template <typename T>
+    struct basic_string{};
+  }
+}
+
+namespace foo {
+  using std::basic_string;
+}
+
+export module std;
+
+export namespace std
+{
+  using std::basic_string;
+}
diff --git a/gcc/testsuite/g++.dg/modules/using-22_b.C b/gcc/testsuite/g++.dg/modules/using-22_b.C
new file mode 100644
index 000000000000..0b66f4ad6b08
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/using-22_b.C
@@ -0,0 +1,13 @@
+// { dg-additional-options "-fmodules-ts" }
+
+import std;
+
+int main()
+{
+  std::basic_string<char> s;
+
+  // The inline namespace should not be exported, only the 'using' in std.
+  std::__cxx11::basic_string<char> s2; // { dg-error "has not been declared" "" { xfail *-*-* } }
+  // The non-exported using should also not be visible.
+  foo::basic_string<char> s3; // { dg-error "has not been declared" "" { xfail *-*-* } }
+}